Numerically controlled machine tools and other apparatus which have to be positioned, particularly when substantial positioning forces are needed, frequently use servo drives or servo motors. For rapid, accurate control, the motors should have a torque output which can be accurately and precisely controlled from lowest speed, in the limiting case from stopped condition, up to the highest maximum rated speed of the motor. The angular torque distribution as the motor operates, should be uniform, so that the circumferential instantaneous speed depends only on the control current. The motor should start from any random angular position after having been stopped.
Usually, d-c motors are used for such applications. Due to the necessary commutation, such motors have substantial disadvantages. It has been proposed to use self-controlled synchronous dynamo electric machines to avoid the disadvantages of d-c motors. Synchronous motors, however, require a positioning transducer which is expensive and introduces additional complexity in order to provide a digital positioning signal which is a precise analog of the position, and thus a high resolution signal.